As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has been also sharply increased as an energy source for the mobile devices. One of the secondary batteries is a lithium secondary battery having high energy density and discharge voltage, on which much research has been carried out and which is now commercially and widely used.
Based on its external shape, the lithium secondary battery is generally classified as a cylindrical battery, a rectangular battery, or a pouch-shaped battery. Based on its electrolyte, the lithium secondary battery is classified as a lithium-ion battery or a lithium-ion polymer battery.
As the mobile devices have been miniaturized, the demand of the rectangular battery and the pouch-shaped battery, which have a relatively small thickness, has increased. A conventional pouch-shaped battery is shown in FIG. 1, which is an exploded perspective view of the conventional pouch-shaped battery, and FIG. 2, which is an assembled perspective view of the conventional pouch-shaped battery.
Referring to FIGS. 1 and 2, the pouch-shaped battery 10 comprises: a pouch-shaped case 20 having an inner space 21 of a predetermined size defined therein; a cover 30 hingedly connected to the pouch-shaped case 20; an electrode assembly 40 including cathode plates 41, anode plates 42, and separators 43, the electrode assembly 40 being located in the receiving part 21 of the pouch-shaped case 20; electrode taps 41a and 42a extending from the ends of the cathode plates 41 and the anode plates 42 of the electrode assembly 40, respectively; and electrode terminals 50 and 51 connected to the electrode taps 41a and 42a, respectively.
At the upper edge of the receiving part 21 of the pouch-shaped case 20 is formed a side extension part 22 having a predetermined width, which is provided for thermal welding. To the middles of the electrode terminals 50 and 51 connected to the electrode taps 41a and 42a is applied a terminal tape 52, which is made of a insulating material to prevent occurrence of short circuit between the electrode terminals 50 and 51 when the side extension part 22 of the pouch-shaped case 20 and a side part 31 of the cover 30 are attached to each other by a thermal welding machine (not shown).
A method of manufacturing the conventional pouch-shaped battery with the above-stated construction will be described below.
First, the electrode assembly 40, which includes the cathode plates 41, the anode plates 42, and the separators 43, is located in the receiving part 21 of the pouch-shaped case 20, and then a predetermined amount of electrolyte is poured into the inner space of the pouch-shaped case 20. At this time, the electrode taps 41a and 42a of the electrode assembly 40 is connected to the corresponding electrode terminals 50 and 51, to the middles of which the terminal tape 52 is applied, and the electrode terminals 50 and 51 and the terminal tape 52 partially protrude from the pouch-shaped case 20 and the cover 30.
Subsequently, the cover 30 is brought into tight contact with the pouch-shaped case 20, and the side extension part 22 of the pouch-shaped case 20 and the side part 31 of the cover 30 are attached to each other in a sealed state by the thermal welding machine (not shown) such that the electrolyte does not leak from the pouch-shaped case 20.
The typical shape of a battery pack having the above-described pouch-shaped battery mounted therein is shown in FIG. 3. FIG. 4 is an exploded perspective view of the battery pack before the battery pack is assembled.
Referring to FIGS. 3 and 4, the battery pack 60 comprises: a rectangular battery 10 having an electrode assembly, which includes cathodes, anodes, and separators, and an electrolyte contained therein in a sealed state; a case body 70 having an inner space to receive the battery 10; and an upper cover 80 attached to the upper part of the case body 70, in which the battery 10 is received, for sealing the battery 10. Between the case body 70 and the battery 10 and between the upper cover 80 and the battery 10 are disposed double-sided adhesive tapes 90.
Generally, the battery pack 60 having the above-stated construction is assembled by attaching the case body 70 and the upper cover 80, which are made of a plastic material, such as polycarbonate (PC) or polyacrylonitrile-butadiene-styrene (ABS), by an ultrasonic welding method. The ultrasonic welding method is a method of welding two surfaces to be attached using frictional heat generated by vibrations of high frequency, for example, 20,000 Hz.
However, as the demand of battery packs having further decreased thicknesses has increased, the thicknesses of the case body 70 and the upper cover 80 are reduced to 0.3 to 0.35 mm. As a result, it is difficult to manufacture the case body and the upper cover by die casting and injection molding. Furthermore, the welding strength is reduced, and therefore, poor welding rate is increased.
In the case that a can is used as the case of the battery, the can provides an adequate strength with regard to the external impacts due to the structural characteristics of the can even though the thickness of the can is small. On the other hand, the pouch-shaped battery 10 as shown in FIG. 1 has a low structural strength with regard to the external impacts. For this reason, the application of the case having a small thickness to the battery pack is restricted.
Furthermore, when the external impacts are applied to the battery pack, the battery 10 is moved upward and downward in the inner space defined between the case body 70 and the upper cover 80, even though the battery 10 and the case body 70 are attached to each other and the battery 10 and the upper cover 80 are attached to each other by the double-sided adhesive tapes 90, with the result that short circuit or suspension of power supply occurs.
For this reason, a battery pack that can be easily manufactured and that has an adequate strength with regard to the external impacts and a high resistance to short circuit or suspension of power supply from the battery pack is highly required.